Wavelength division multiplexing is the transmission of several different signals via a single optical transmission medium (e.g. fibre), by sending each signal (“channel”) at a different optical frequency or wavelength. A multiplexer is used to combine the different channels together for transmission, and a demultiplexer is used to separate the channels following transmission. WDM optical transmission systems are typically composed of a number of spans of optical fibre linking together the network nodes.
Early WDM networks used simple, fixed optical filters to route the optical signals between the network nodes. Such networks were therefore essentially “static” i.e. the channel configuration (number of channels being transmitted, and the routing of the channels through nodes of the network) did not change, except during fault conditions or due to human intervention to upgrade or alter the network configuration.
More recent WDM networks can include reconfigurable optical network nodes, which allow remote reconfiguration of the channels, faster provisioning of new channels and improved network resilience. Such reconfigurable optical network nodes commonly employ integrated optical devices, such as ROADM (Reconfigurable Optical Add-Drop Multiplexer) or WSS (Wavelength-Selective Switch) devices or similar, in order to control and route the optical signals. Although discrete optical devices can be used to provide the same functionality, generally it is cheaper to utilise integrated devices for high channel-count WDM networks.
Autonomous rerouting of traffic is relatively well known in single-channel optical networks such as SDH (Synchronous Digital Hierarchy) where it is commonly known as ASTN (Automatically Switched Transport Network). In the more advanced versions of WDM networks, the network control software can also autonomously reroute traffic according to the demands on the network e.g. faults, changing bandwidth requirements etc, thus reducing the need for human intervention. Such a WDM network is commonly referred to as ASON (Automatically Switched Optical Network).
In general, signal regeneration using optical-electronic-optical conversion is not performed at every node, but only where necessary to restore signal quality. Hence, the routing and transmission of signals from node to node is generally performed in the optical domain.
It is desirable to control the power levels of each of the channels in an optical system. Optical signals experience wavelength dependent effects as they are transmitted across the network, including fibre and optical device attenuation, Raman scattering within the optical fibre, optical amplifier wavelength dependent gain etc. It is particularly desirable to control the optical signal power as channels are added to the network, to prevent disruption to existing established channels or radiation power surges which can interfere with existing channels e.g. due to the channels being amplified by non-linear devices such as erbium doped fibre amplifiers.
FIG. 1 is a schematic diagram of a device 10 suitable for use in an ASON node, for wavelength switching and control of optical power.
The device 10 comprises a port selector switch 12 which is operable to couple any one of a plurality of input ports 14a, 14b . . . 14m to a Variable Optical Attenuator (VOA) 16. The port selector switch 12 thus acts to select an optical signal path by switching to a particular port, thus directing an optical signal along a desired route through the network. The VOA 16 controls the power of the optical signal transmitted to the output port 18, keeping the channel power within an acceptable dynamic range.
In order to keep the optical signal within the acceptable dynamic range, the VOA 16 will be coupled to a controller 20. A power monitor function is provided at each node which continuously measures the individual channel powers at each line port (e.g. at line port 18). Such monitoring is usually carried out only after the VOA for cost reasons. The controller 20 determines an error signal based on the difference between the measured channel power and a target channel power (e.g. a transmission power level suitable for normal transmission of the optical channel within the network), with a fixed multiple of the determined error signal (the gain) then used to control the VOA attenuation. The controller 20 is often implemented as a cyclic control algorithm. Adjustments are made to the appropriate VOA attenuation at predetermined regular intervals. The period between one VOA update and the next is termed the control cycle.
FIG. 2 shows the typical progression of optical channel power at the output of a node, as a channel is introduced to the node. The horizontal axis shows time on an arbitrary scale and the vertical axis shows optical power on an arbitrary decibel (dB) scale.
At the beginning of the process (state A) the VOA 16 is at maximum attenuation (in the blocking state) causing the channel power at the output of the device (and hence the node) to be negligible. This is typically the quiescent state of the device, in order to minimise the transmission of optical noise power through the node. Such noise can be produced during optical amplification of signals at upstream nodes.
To introduce the channel, the VOA 16 is first adjusted to a predetermined “safe” value (state B in FIG. 2) in order to perform an initial calibration step. This step is necessary as the optical power is monitored only after the VOA 16, and therefore the channel power at the input to the VOA is unknown. If the input power of the optical signal supplied to the node was unexpectedly high and the VOA was subsequently set to a low attenuation, the resulting very high output power of the signal from the node could cause a disturbance to pre-existing channels along the channel route, and even damage the downstream optical detectors. Hence, this initial “safe” attenuation value of the VOA is chosen such that it results in an acceptably low channel output power after taking into account the various component tolerances and likely range of input powers. Once the VOA has been set to this “safe” attenuation value, the optical output power is measured in order to check that the input power is within an acceptable range to allow subsequent correct operation of the node.
Once the initial calibration step has been performed by reading the channel output power, the control loop algorithm is activated, and acts to reduce the VOA attenuation, producing the steps seen in time interval C in FIG. 2. Thus, the power of the channel gradually approaches the target output power, with the change in channel power for each step decreasing as the target channel power is approached, until the channel power at the node output substantially reaches the target channel power (i.e. state D). It will be seen that, in this case, the output power takes about 20 steps (i.e. about 20 control cycles) to move from the safe value (state B) to the target value (state D). Subsequently, the same control loop algorithm will act to control the VOA attenuation such that the optical power output is maintained at, or close to, the target channel power.
An optical channel will typically be transmitted along an optical link comprising several nodes. To prevent disruption to existing established channels or radiation power surges which can interfere with existing channels, a new channel will be iteratively introduced to each successive node. Once the channel power is substantially at the target channel power (i.e. state D in FIG. 2) at a node, then that node will transmit a signal to the next downstream node. The VOA at that downstream node will then start the process of introducing the channel at that downstream node. For example, the relevant signal indicating that a channel power at a node output has substantially reached the target channel power can be transmitted via the optical supervisory channel (OSC), using an “equipped wavelength table”. The equipped wavelength table is data indicating which wavelength channels are fully present (i.e. at normal output power from a node). An update to the equipped wavelength table to indicate that a new channel is now fully present can also be termed a “channel present” message.